Generally, a magnetic recording medium such as a magnetic tape is produced by coating and drying a magnetic coating liquid comprising a magnetic powdery material, binder resin, and the like on a support.
In such a magnetic recording medium, particularly in video tape, carbon black is used in combination with the above materials in order to satisfy simultaneously some of the properties such as the conductivity, coefficient of friction, surface nature (i.e., electromagnetic conversion property), wear resistance, and the like. As the technique of this kind there are known those conventional techniques disclosed in, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 5426/1984, 1614/1984, 218039/1983 and 211321/1983, and Japanese Patent Examined Publication Nos. 20203/1978, 9041/1979 and 4968/1982.
These techniques, however, were inadequate to provide a recording medium having an excellent conductivity and a low coefficient of friction, i.e., being excellent in the runnability as well as in the wear resistance, without roughening the surface, i.e., without deteriorating the electromagnetic conversion property.
Namely, in the above-mentioned techniques, where carbon black whose particle sizes are not more than 30 m.mu. were used, the conductivity is not always improved. For example, carbon black with a particle size of 23 m.mu., when used along, must be incorporated into the magnetic layer in a quantity of not less than 15 parts by weight to 100 parts by weight of a magnetic powdery material to make the surface resistivity lower than 10.sup.10 .OMEGA./sq. However, if such a large amount of carbon is added, the electromagnetic conversion property is lowered. Also, in the carbon black whose particle sizes are not more than 40 m.mu., the friction on the magnetic layer's surface increases. If, in order to reduce the friction, a large amount of a lubricant is added, then there occur such phenomena as the deterioration of the electromagnetic conversion property, bleed-out phenomenon, etc. Where a large amount of carbon black whose particle sizes are not less than 50 m.mu. is used, the electromagnetic conversion property is lowered. Also, the carbon black whose particle sizes are not less than 50 m.mu. do not always improve the electromagnetic conversion property but rather generally lower the electromagnetic conversion property as in the case of carbon black having particle sizes of not more than 30 m.mu. and a BET value of not less than 500 m.sup.2 /g or carbon black having particle sizes of not less than 40 m.mu..